catflap.org Online Dictionary Query

  be alert for trouble /biː ɐlˈɜːt fɔː tɹˈʌbəl/
  auf der Hut sein, wachsam sein
     Synonyms: be on (the) alert, be alert, be on the qui vive
  
   see: stiff hat, hat, titfer, stiff hats, hats, titfers, bucket hat, cloche hat, cloche, panama hat, panama, slouch hat, Tyrolean hat, Hats off to him!, You may just keep it (I don't want to have it)., unite different interest, be on one's guard, look out for squalls